Developers of modern thrusters have long sought to achieve increased reliability, performance and flexibility of operation for such thrusters. Two principal thruster designs are utilized, those which utilize liquid propellants, and those which utilize solid propellants.
Thrusters that utilize liquid propellants are capable of controlled thrust, including the ability to stop and restart on command. Though the system reliability is generally acceptable, the complexity associated with the many moving parts, such as valves, required for their operation creates significant additional design and construction expense. In addition, the majority of liquid propulsion systems utilize propellants that are toxic to humans.
Although the technology of liquid propellant thruster systems is mature and proven, room for improvement exists. In U.S. Pat. No. 5,533,331, Campbell, et al. disclose the state-of-the-art for non-toxic liquid propulsion systems, with a focus on missile divert thrusters and attitude control thrusters. Although the liquid propellant system disclosed by Campbell et al. enable full thrust control over a wide range of values, are completely capable of many start/stop/restart cycles, and further provide a solution to the toxicity issue, the system is still encumbered by complex plumbing hardware.
In contrast, thrusters that utilize solid propellants offer improved reliability relative to the counterpart liquid propellant thrusters as a result of their reduced complexity. However, the penalty is that solid propellant thruster systems are unable to simply throttle output thrust in accordance with real-time commands by the user.
For example, one mechanical means of controlling thrust of such a solid propellant thruster is to manufacture the thruster in such a way as to intentionally produce void space within the grain of the solid propellant. The function of such void space being to alter the volume in which the combustion product gasses may expand, thereby changing the pressure and propellant burn rate and thrust. The obvious limitation of such techniques is that the thrust profile is fixed at the time of manufacture, with no provision for real-time alteration of thrust level. Examples of such systems are provided in U.S. Pat. No. 4,357,795 to Bastian, et al. and U.S. Pat. No. 3,023,570 to Crouch.
Although such grain voids may be able to control the rate of thrust, systems that achieve multiple restarts for solid propellant thrusters tend to focus on discrete segments of propellant charges separated by some form of frangible material and separate ignition means. For example, U.S. Pat. No. 4,972,673 to Carrier, et al. provides a dual stage solid propellant rocket motor that relies upon a physical barrier between two distinct propellant grain regions to be broken upon command. However, problems arise in designing a barrier material that is capable of maintaining insulating integrity without fail before the command to ignite the protected segment, and yet will fracture with certainty when the command is made. Despite this inherent problem, development of these membrane separation solid propellant thrusters continues, such as, for example, in U.S. Pat. No. 5,675,966 to Dombrowski, et al.
A particularly innovative means of thrust control that also achieves the zero thrust, start/stop multicycle operation is disclosed in McDonald, et al.'s U.S. Pat. No. 4,840,024, and U.S. Pat. No. 5,808,231 to Johnston, et al., in which a complex system of valves and ducting is employed to control thrust. Specifically, in such a system the pressure within the combustion chamber may be suddenly reduced by appropriate valve operation such that the solid propellant extinguishes. Reignition is accomplished by another valve operation sequence that introduces gas generator products into the combustion chamber, thus allowing for an increase in pressure and temperature sufficient to cause propellant reignition. However, a significant problem associated with this solution to the problem of thrust control and restartability for solid propellant thrusters is the extreme complexity of the valving apparatus and the significant increase in total system mass.
Another creative solution to solid propellant thruster control is disclosed by Kosaka, et al. in U.S. Pat. No. 4,397,149, in which they disclose the invention of a multi grain solid propellant thruster system, whereby increased thrust is achieved when a grain that is not yet ignited is mechanically moved into position to ignite and thus augment thrust. Although a variety of such mechanical means have been proven successful for controlling the thrust of solid propellant thrusters, such as by altering the nozzle throat area, as McCullough indicates in U.S. Pat. No. 3,989,191, most of these designs suffer from the erosion of the moving parts exposed to the extreme environment of the combustion zone. For example, Knapp, et al., in U.S. Pat. No. 5,491,973 disclose a pintle assembly which is self-actuated to vary the nozzle throat area and thus alter thrust. However, erosion of the mechanism is a serious problem and no provision for complete extinguishment with subsequent restart exists.
Finally, another novel approach to controlling thrust by variations in the combustion gasses exit area is disclosed by Douglass, et al. in U.S. Pat. No. 4,550,888 in which a series of valves capable of surviving high temperature are placed in parallel fashion with the combustion chamber. Changes in thrust are achieved by altering the number of exhaust valves that are open, effectively changing the exit area; however, no provision for extinguishing and then reigniting the propellant exists.
Between the two extreme classes of thruster systems is the hybrid thruster, which is an attempt to combine the advantages of the solid and liquid thruster systems typically by using a solid fuel and an injected liquid oxidizer to ignite the propellant in a hypergolic fashion. Although the complexity of these systems is cut in half, the hybrid system still requires complex valves and generally toxic propellants.
A thorough description of the prior art of hybrid thruster systems is found in U.S. Pat. No. 5,715,675 to Smith, et al. and also in U.S. Pat. No. 5,099,645 to Schuler, et al. Smith, et al. clearly state that the most significant problem associated with hybrid thrusters is the remarkable complexity and difficulty associated with achieving successful and consistent thruster operation. Kline, et al. in U.S. Pat. No. 6,125,763 disclose an invention that utilizes the majority of advances in the state-of-the-art, e.g. a membrane for the purpose of insulating the solid propellant charge until such time as it is needed to ignite, a valve to control combustion chamber pressure, a liquid propellant feed system to throttle thrust, etc. Again, such a system, although providing all of the needed functionality for a thruster requires a very complicated thrusting system.
Accordingly, although state-of-the-art thrusters capable of throttling thrust, including multiple start/stop/restart cycles exist, each system suffers significant drawbacks, and, as such, a need exists for a simple, inexpensive, reliable thruster design capable of throttling thrust, including multiple start/stop/restart cycles.